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soil. Soils that are allowed to age on the metal surface for an extended time become increasingly difficult to remove. A classic example is aged buffing compounds on zinc die castings. It is important, therefore, to clean parts soon after they arrive from their last operation. PROPRIETARY CLEANERS To begin with the concept of one cleaner for all soils and all metals does not exist, although chlorinated solvents have come close to achieving that honor. Proprietary cleaners fall into the following groups: alkaline (mild to strong); neutral (pH 7.0); acidic (mild to strong); emulsion; and solvent. The physical forms of cleaners on the market may be powder, liquid, or gel. In the development of a cleaner not only are the soils a consideration but also the base metal to be cleaned. As a rule the cleaner's function is to remove the soil and not have any detrimental effect on the metal's surface. For example, a prod- uct containing caustic soda would be satisfactory for cleaning ferrous metals but not for cleaning aluminum alloys, zinc die castings, galvanized stock, or yellow brass. Such a product would attack these nonferrous surfaces. The proper selec- tion of a cleaner for the metal substrate to be cleaned is thus paramount. The bulk of the proprietary cleaners used in industry fall into the alkaline group. Many solvent cleaners are being phased out because of environmental con- siderations and other hazards. Acid cleaners generally are used for cleaning stainless steel alloys, wrought aluminum alloys, copper, and brass alloys. Proprietary alkaline soak and spray cleaners are generally formulated to clean a variety of metal-forming lubricants from a metal's surface, and may also find application for cleaning a variety of metals, i.e., ferrous metals, aluminum alloys, brass, and magnesium alloys. Within the last three years the development of a new cleaner formulation has www.metalfinishing.com/advertisers 65